This newest volume in the Weight Watchers series will in general serve the interests of those dedicated to the diet program--but a few caveats are in order. More than 250 recipes appear along with nutritional information, including the portion sizes that correspond to the Weight Watchers system. At a glance, the recipes sound intriguing (and not what one would expect from a diet book): a crusty roast duck with cherry relish; a hearty vegetable risotto; a prodigally rich mocha pudding pie. Still, not all are practical, and inconsistencies mar the pages. Some recipes call for sour cream, others for reduced-calorie sour cream. Many rely on hard-to-locate ingredients such as 1% buttermilk, evaporated skim milk and instant polenta (or quick-cooking cornmeal). Additionally, readers are urged to quick-cook beets and shrimp in the microwave, yet instructions are not given. Because recipes are designed to yield a small number of portions, they utilize tiny amounts of ingredients without cluing us in as to how to preserve or store what's inevitably left over. Self-sabotage is the dieter's plague; surprisingly, Weight Watchers doesn't acknowledge its potential. Photos not seen by PW. (Jan.)